Hi Vinz ...
Let's do it step by step:
Dep: 1505 (by R921) | FREE
of supplements as it is a regional train
This is ONE travel day.
Dep: 0822 (by IC train) | FREE
of supplements like mentioned here in the rail.cc/en/interrail-train-reservation/finland/fi
supplements list for Finland
Helsinki Dep: 1926 | supplements - see link bellow
Rovaniemi Arr: 0753
Helsinki Dep: 2221 | supplements - see link bellow
Rovaniemi Arr: 1041
You will need TWO travel days, ONE for the morning train Jyvaskyla to Helsinki, and ONE for the night train Helsinki to Rovaniemi. You can NOT use the rule of the night trains 1900 in reverse!! rail.cc/en/interrail-night-train
The supplement for a seat in these night trains is at least EUR 4,60 - for more details: rail.cc/en/helsinki-turku-kemi-rovaniemi-night-train/f2948
To book a seat or bed in a night train, either do it at a station in Finland or phone them - number is mentioned at the link for the night train above.
In your case you could get your reservations for all trains already in Turku at the beginning of your tour.
Rovaniemi Dep: 1630 (by regional train) | FREE
| On this connection you use the TRAVEL DAY of the 15th already used for the night train Helsinki-Rovaniemi
Kemi Arr: 1746
Kemi Dep: 2234 | supplements needed for a seat - see link above for the night trains
Tampere Arr: 0548
This is ONE travel day. ONE for Rovaniemi-Kemi (you use the 15th, already used for the night train Helsinki-Rovaniemi) and then ONE for the night train down to Tampere.
You have in total FOUR travel days used.
And if you like this help, please order your official InterRail ticket via the webshop interrail.rail.cc
- you then support this forum and all the work ... thank you!!
Have fun, Peter